1. Field of the Invention
The present invention relates to a reference frequency control circuit.
2. Description of the Related Art
Common electronic equipment includes, for example, a crystal oscillator circuit for generating a high precision frequency serving as a reference frequency in the electronic equipment. A crystal oscillator circuit 100 exemplary shown in FIG. 2 is a circuit in which an amplifier circuit 301 including an inverter 203 and a resistor 204 is connected to a resonant circuit 300 including a quartz crystal 200 and capacitors 201 and 202 via a resistor 205 so as to oscillate at a predetermined frequency (see, e.g., Japanese Patent Application Laid-Open Publication Nos. 2004-140817 or 2006-287765). The resistor 205 is a resistor for preventing abnormal oscillation in the crystal oscillator circuit 100.
Since the resonant circuit 300 has a large high Q factor in general, the crystal oscillator circuit 100 is capable of generating a high precision oscillation frequency. However, if the temperature of the surroundings of the crystal oscillator circuit 100 changes, the characteristics of the quartz crystal 200 change, for example, and thus, the resonant frequency of the resonant circuit 300 changes. Since the oscillation frequency of the crystal oscillator circuit 100 is equal to the resonant frequency thereof in general, the oscillation frequency of the crystal oscillator circuit 100 also changes according to the temperature, as a result. Even if the temperature is constant, the resonant frequency varies due to the influence of unevenness in manufacturing the capacitors 201 and 202, for example. Therefore, oscillation frequencies vary among different crystal oscillator circuits 100. Variations in the oscillation frequencies according to the temperature as above can be suppressed by using a TCXO (Temperature Compensated Crystal Oscillator), for example. Variations in the oscillation frequency due to the influence of unevenness in manufacture, however, cannot be suppressed. In addition, the TCXO is expensive in general.